Unit Overview
Physical quantities and units
CIE A-Level PhysicsΒ· 5 min read π n/a
1. Unit at a glance
This unit builds the fundamental toolkit you need to work with and communicate physics concepts correctly. We start with how we classify quantities, then move to unit conventions, before covering practical skills like estimation and uncertainty analysis. Every calculation, experiment and problem in the rest of your course relies on the conventions learned here.
This unit is split into 6 connected sub-topics covering theory and practical foundational skills:
Types of physical quantity
Learn how we categorize physical quantities for consistent physical analysis.
β β± 3 min
Scalars and vectors
Distinguish between scalar and vector quantities, and learn basics of vector combination.
β β β± 5 min
SI base and derived units
Understand the SI system, base units, and how to derive units for compound quantities.
β β± 4 min
Unit prefixes
Convert between different magnitudes of units using standard SI prefixes.
β β± 3 min
Estimation of physical quantities
Practice estimating the order of magnitude of common everyday physical quantities.
β β β± 4 min
Measurement uncertainties
Calculate and combine absolute, fractional and percentage uncertainty for experiments.
β β β β± 7 min
2. Common Pitfalls
Wrong move:
Confusing scalar (magnitude only) and vector (magnitude + direction) quantities.
Why:
This leads to incorrect calculations when combining quantities, especially in mechanics.
Correct move:
Always explicitly check if a quantity has direction before classifying it.
Wrong move:
Using mismatched units when substituting values into formulas.
Why:
Mismatched units lead to drastically incorrect results even if your method is correct.
Correct move:
Convert all quantities to base SI units before starting any calculation.
Wrong move:
Adding absolute uncertainties when multiplying quantities, or percentage uncertainties when adding.
Why:
This produces incorrect uncertainty estimates for final experimental results.
Correct move:
Add percentage uncertainties for multiplication/division; add absolute uncertainties for addition/subtraction.
3. Quick Reference Cheatsheet
Concept / Rule | Description |
|---|---|
Scalar quantity | Physical quantity with magnitude only, no direction |
Vector quantity | Physical quantity with both magnitude and direction |
SI base units | 7 fundamental units: kilogram (kg), metre (m), second (s), ampere (A), kelvin (K), mole (mol), candela (cd) |
Order of magnitude | The nearest power of 10 to the value of a physical quantity |
Percentage uncertainty | |
Combined uncertainty rule | Add % uncertainties for Γ/Γ·; add absolute uncertainties for +/β |
What's Next
Start your study of this unit with the first sub-topic, Types of physical quantity, to build your foundational understanding of how we categorize quantities in physics. Once you complete all sub-topics in this unit, you will move on to the next unit of CIE 9702 A-Level Physics, which covers kinematics, the study of motion.